III-Nitride Crystal Manufacturing Method, III-Nitride Crystal Substrate, and III-Nitride Semiconductor Device

ABSTRACT

Affords methods of manufacturing bulk III-nitride crystals whereby at least the surface dislocation density is low globally. 
     The present III-nitride crystal manufacturing method includes: a step of preparing an undersubstrate ( 1 ) containing a III-nitride seed crystal, the III-nitride seed crystal having a matrix ( 1   s ), and inversion domains ( 1   t ) in which the polarity in the &lt;0001&gt; directions is inverted with respect to the matrix (1 s ); and a step of growing a III-nitride crystal ( 10 ) onto the matrix ( 1   s ) and inversion domains ( 1   t ) of the undersubstrate ( 1 ) by a liquid-phase technique; and is characterized in that a first region (10 s ), being where the growth rate of III-nitride crystal ( 10 ) growing onto the matrix (1 s ) is greater, covers second regions ( 10   t ), being where the growth rate of III-nitride crystal ( 10 ) growing onto the inversion domains ( 1   t ) is lesser.

TECHNICAL FIELD

The present invention relates to methods of manufacturingGroup-III-nitride crystals of low dislocation density, to III-nitridecrystal substrates obtained by the manufacturing methods, and toIII-nitride semiconductor devices incorporating the III-nitride crystalsubstrates.

BACKGROUND ART

III-nitride crystals find applications in light-emitting devices such aslight-emitting diodes (LEDs) and laser diodes (LDs), in electronicdevices such as rectifiers, bipolar transistors, field-effecttransistors, and high-electron-mobility transistors (HEMTs), insemiconductor sensors such as temperature sensors, pressure sensors,radiation sensors, and visible-blind ultraviolet detectors, insurface-acoustic-wave devices (SAW devices), in acceleration sensors, inmicro-electromechanical system parts (MEMS parts), in piezoelectricvibrators, in resonators, and in piezoelectric actuators. III-nitridecrystal of low dislocation density is being sought after in suchapplications, in order to improve the performance characteristics of thevarious devices.

Proposed methods of manufacturing III-nitride crystal to have lowdislocation density include the following techniques. JapaneseUnexamined Pat. App. Pub. No. 2003-183100 (“Patent Document 1”hereinafter) discloses a facet-growth method in which a mask patternedin regular stripes is provided on an undersubstrate, and atop the maskGaN crystal is vapor-deposited while forming plurally faceted, linearV-grooves and sustaining the grooves, and whereby dislocations withinthe GaN crystal are gathered directly beneath the V-grooves (the regionsinto which dislocations concentrate being termed “crystal-defectgathering regions”), reducing the surrounding dislocation density.

With the just-described facet growth method of Patent Document 1,dislocation density in the region apart from the crystal-defectgathering regions can be reduced to a level of 1×10⁵ cm⁻², yet thedislocation density in the crystal-defect gathering regions will behigh. Moreover, in such defect-gathering regions, the polarity in the<0001> directions is often inverted with respect to the crystal regionapart from the regions where defects gather. The consequent challengesof epitaxially growing a III-nitride semiconductor layer onto a GaNcrystal substrate obtained by facet growth mean low semiconductor deviceyields.

Meanwhile, Japanese Unexamined Pat. App. Pub. No. H10-312971 (“PatentDocument 2” hereinafter) discloses an epitaxial lateral overgrowth (ELO)technique in which an undersubstrate being a GaN thin film formed onto,e.g., sapphire is prepared, from atop the undersubstrate a mask of,e.g., SiO₂, having apertures is formed, and GaN crystal is epitaxiallygrown laterally onto the mask through the apertures.

The just-described ELO technique of Patent Document 2 makes lateralcrystal growth without occurrences of strain and cracking possible, thusreducing dislocation density by comparison with implementations in whichGaN crystal is grown directly onto an undersubstrate; yet freshdislocations arise where the laterally growing crystal coalesces, whichis prohibitive of getting the dislocation density down to under 1×10⁷cm⁻². On this account, making such GaN substrates practicable assubstrates for LDs has been problematic.

In another proposal, the detailed description in U.S. Pat. No. 5,868,837(“Patent Document 3” hereinafter) discloses a sodium flux technique inwhich, at a temperature of some 600° C. to 800° C. and under a nitrogenatmosphere at a pressure of some 5 MPa, nitrogen gas is supplied to aGa—Na melt to grow GaN crystal.

The just-described sodium flux method of Patent Document 3 makes itpossible to grow low-dislocation-density, low-defect GaN crystal undertemperature and pressure conditions relatively moderate for aliquid-phase technique, yet the crystal growth rate is slow, which isprohibitive of obtaining bulk GaN crystal.

In still another proposal, Japanese Unexamined Pat. App. Pub. No.2004-221480 (“Patent Document 4” hereinafter) discloses forming astarting substrate, in which differing polarity A, B domains coexist,into a skeletal substrate in which the entirety or a portion of eitherone of the polarity domains has been removed by etching, and by growingonto the skeletal substrate crystal of the same material as thesubstrate, filling in the given removed portion with crystal having theother polarity, to obtain crystal whose entire surface has the otherpolarity. With the method of Patent Document 4, however, because thedomains of the one polarity are domains in which the polarity in the<0001> directions is inverted with respect to the domains of the otherpolarity, when those domains are filled in by crystal growth using avapor-phase technique, growth in which the polarity of those domains(the one polarity) is inherited occurs. Thus, to the extent that theentire crystal surface is to be covered with crystal having the otherpolarity, the entirety or a portion of the domains of the one polarityin the substrate must be deeply removed by etching, which complicatesthe manufacturing method.

Patent Document 1: Japanese Unexamined Pat. App. Pub. No. 2003-183100.Patent Document 2: Japanese Unexamined Pat. App. Pub. No. H10-312971.Patent Document 3: Detailed Description in U.S. Pat. No. 5,868,837.Patent Document 4: Japanese Unexamined Pat. App. Pub. No. 2004-221480.

DISCLOSURE OF INVENTION Problem Invention is to Solve

An object of the present invention is to resolve the problems discussedabove by making available methods of manufacturing bulk III-nitridecrystal whereby at least the surface dislocation density is lowglobally, and to make available III-nitride crystal substrates obtainedby the manufacturing methods and III-nitride semiconductor devicesincorporating the III-nitride crystal substrate.

Means for Resolving the Problems

The present invention is a III-nitride crystal manufacturing methodincluding: a step of preparing an undersubstrate containing aIII-nitride seed crystal, the III-nitride seed crystal having a matrix,and inversion domains in which the polarity in the <0001> directions isinverted with respect to the matrix; and a step of growing a III-nitridecrystal onto the matrix and inversion domains of the undersubstrate by aliquid-phase technique; characterized in that a first region, beingwhere the growth rate of III-nitride crystal growing onto the matrix isgreater, covers second regions, being where the growth rate ofIII-nitride crystal growing onto the inversion domains is lesser.

In a III-nitride crystal manufacturing method involving the presentinvention, the undersubstrate may be one in which the surface of theinversion domains is recessed relative to the matrix surface.Furthermore, the inversion domains can be in the form of a plurality ofstripe regions along a {0001} plane on the undersubstrate, with thestriped regions being arranged parallel to each other at regularintervals. Optionally, the inversion domains can be in the form of aplurality of dot regions along a {0001} plane on the undersubstrate,with the dotted regions being arranged two-dimensionally at regularintervals. Also contemplated is that the inversion domains are in ahoneycomb form, arranged two-dimensionally in closed-packed regularhexagons along a {0001} plane on the undersubstrate.

Furthermore, in a III-nitride crystal manufacturing method involving thepresent invention, the III-nitride crystal may be grown to a thicknessof 1 μm or more. Moreover, in its surface the III-nitride crystal canhave a resistivity of 1×10⁵ Ω·cm or more. Also, the III-nitride crystalcan be grown in a nitride reaction vessel.

In addition, a step of causing the III-nitride crystal that has beengrown onto an undersubstrate by a liquid-phase technique to grow furtherby a vapor-phase technique may also be included a III-nitride crystalmanufacturing method involving the present invention.

The present invention is furthermore a III-nitride crystal substrateobtained by processing III-nitride crystal produced by a manufacturingmethod described above. Herein, the processing of the III-nitridecrystal may include a step of slicing or cleaving the III-nitridecrystal. The present invention is also a III-nitride semiconductordevice in which an at least single-lamina III nitride semiconductorlayer is formed onto the just-noted III-nitride crystal substrate.

EFFECTS OF THE INVENTION

The present invention affords methods of manufacturing bulk III-nitridecrystal whereby at least the surface dislocation density is lowglobally, affords III-nitride crystal substrates produced by themanufacturing methods, and III-nitride semiconductor devicesincorporating the III-nitride crystal substrates.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A, in a sectional outline diagram illustrating one embodiment modeof a III-nitride crystal manufacturing method involving the presentinvention, represents a step of preparing an undersubstrate.

FIG. 1B, in a sectional outline diagram illustrating the one embodimentmode of a III-nitride crystal manufacturing method involving the presentinvention, represents a step of growing a III-nitride crystal by aliquid-phase technique.

FIG. 2A, in a sectional outline diagram illustrating another embodimentmode of a III-nitride crystal manufacturing method involving the presentinvention, represents a step of preparing an undersubstrate.

FIG. 2B, in a sectional outline diagram illustrating this otherembodiment mode of the III-nitride crystal manufacturing methodinvolving the present invention, represents a step of growing aIII-nitride crystal by a liquid-phase technique.

FIG. 3 is an outline plan view representing one example of an inversiondomain layout in an undersubstrate utilized in a III-nitride crystalmanufacturing method involving the present invention.

FIG. 4 is an outline plan view representing another example of aninversion domain layout in an undersubstrate utilized in a III-nitridecrystal manufacturing method involving the present invention.

FIG. 5 is an outline plan view representing a further example of aninversion domain layout in an undersubstrate utilized in a III-nitridecrystal manufacturing method involving the present invention.

FIG. 6 is an outline plan view representing still another example of aninversion domain layout in an undersubstrate utilized in a III-nitridecrystal manufacturing method involving the present invention.

FIG. 7 is an outline plan view representing a still further example ofan inversion domain layout in an undersubstrate utilized in aIII-nitride crystal manufacturing method involving the presentinvention.

FIG. 8 is an outline plan view representing yet another example of aninversion domain layout in an undersubstrate utilized in a III-nitridecrystal manufacturing method involving the present invention.

FIG. 9 is an outline plan view representing an even further example ofan inversion domain layout in an undersubstrate utilized in aIII-nitride crystal manufacturing method involving the presentinvention.

FIG. 10A, in a sectional outline diagram illustrating a furtherembodiment mode of a III-nitride crystal manufacturing method involvingthe present invention, represents III-nitride crystal grown onto anundersubstrate by a liquid-phase technique.

FIG. 10B, in a sectional outline diagram illustrating this furtherembodiment mode of a III-nitride crystal manufacturing method involvingthe present invention, represents a step of forming III-nitride crystalsubstrates.

FIG. 11A, in a sectional outline diagram illustrating yet anotherembodiment mode of a III-nitride crystal manufacturing method involvingthe present invention, represents III-nitride crystal grown onto anundersubstrate by a liquid-phase technique.

FIG. 11B, in a sectional outline diagram illustrating this yet otherembodiment mode of a III-nitride crystal manufacturing method involvingthe present invention, represents a step of causing the III-nitridecrystal that has been grown by a liquid-phase technique to grow furtherby a vapor-phase technique.

FIG. 11C, in a sectional outline diagram illustrating this yet otherembodiment mode of a III-nitride crystal manufacturing method involvingthe present invention, represents a step of forming III-nitride crystalsubstrates.

FIG. 12 is a sectional outline diagram illustrating one example of aIII-nitride semiconductor device involving the present invention.

FIG. 13 is a sectional outline diagram illustrating another example of aIII-nitride semiconductor device involving the present invention.

LEGEND

-   -   1: undersubstrate    -   1 f: squares    -   1 g: regular triangles    -   1 h: regular hexagons    -   1 s: matrix    -   1 t: inversion domains    -   1 t _(a), 1 t _(b): striped domains    -   1 t _(m), 1 t _(n): dotted domains    -   2: crystal-growth solution    -   3: nitrogen-containing gas    -   7: reaction vessel    -   10: III-nitride crystal    -   10 s: first region    -   10 t: second regions    -   10 v: vapor-deposited region    -   100: III-nitride crystal substrate    -   110, 120: III nitride semiconductor layers    -   111: n-type Al_(0.1)Ga_(0.9)N cladding layer    -   112: n-type GaN guide layer    -   113: multiquantum-well active layer    -   114: p-type Al_(0.2)Ga_(0.8)N protective layer    -   115: p-type GaN guide layer    -   116: p-type Al_(0.1)Ga_(0.9)N cladding layer    -   117: p-type GaN contact layer    -   118: p-side electrode    -   119: n-side electrode    -   121: undoped Al_(0.26)Ga_(0.74)N spacer layer    -   122: Si-doped n-type Al_(0.26)Ga_(0.74)N carrier supply layer    -   123: Si-doped n-type GaN contact layer    -   125: gate electrode    -   126: source electrode    -   127: drain electrode

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment Mode 1

Referring to FIG. 1, one embodiment mode of a III nitride crystalmanufacturing method involving the present invention includes: a step(FIG. 1A) of preparing an undersubstrate 1 containing a III nitride seedcrystal, the III nitride seed crystal having a matrix 1 s and inversiondomains 1 t in which the polarity in the <0001> directions is invertedwith respect to the matrix 1 s; and a step (FIG. 1B) of growing a IIInitride crystal 10 onto the matrix 1 s and inversion domains 1 t of theundersubstrate 1 by a liquid-phase technique. Herein, this embodimentmode is characterized in that a first region 10 s, in which the growthrate of the III nitride crystal 10 growing onto the matrix is greater,covers second regions 10 t in which the growth rate of the III nitridecrystal 10 growing onto the inversion domains 1 t is lesser.

In Embodiment Mode 1, the III-nitride crystal 10 is grown by aliquid-phase technique onto the undersubstrate 1 containing theIII-nitride seed crystal having the matrix 1 s and inversion domains 1 tin which the polarity in the <0001> directions is inverted with respectto the matrix 1 s. Specifically, to begin with the undersubstrate 1 isplaced in a reaction vessel 7, and a crystal-growth solution 2containing a Group-III element is formed surrounding the undersubstrate1. Next, a nitrogen-containing gas 3 is supplied to the crystal-growthsolution 2 to grow the III-nitride crystal 10 onto the undersubstrate 1.Alternatively, a nitrogen-containing substance (for example, a IIInitride) may be dissolved into the crystal-growth solution 2 to growIII-nitride crystal onto an undersubstrate.

In the undersubstrate 1, the III-nitride seed crystal is manufactured,for example, as described in paragraphs [0221] through [0271] of PatentDocument 1, by a facet growth method by means of hydride vapor phaseepitaxy (HVPE), to lower the dislocation density in the matrix 1 s ofthe III-nitride seed crystal, with the dislocation density in theinversion domains 1 t of the III-nitride seed crystal being made highercompared with that in the matrix 1 s. Herein, FIG. 1 represents thesituation in which the undersubstrate 1 is III-nitride crystal formed ofthe matrix 1 s and inversion domains 1 t, but a (not-illustrated)undersubstrate in which III-nitride crystal composed of a matrix 1 s andinversion domains 1 t is formed onto a normative substrate other thanIII-nitride crystal may also be utilized.

In the III-nitride crystal 10, the first region 10 s, growing onto thematrix 1 s of the undersubstrate 1, grows as crystal inheriting thepolarity of, and low dislocation density in, the matrix 1 s. Meanwhilein the III-nitride crystal 10, the second regions 10 t, growing onto theinversion domains 1 t of the undersubstrate 1, grow as crystalinheriting the polarity of, and high dislocation density in, theinversion domains 1 t. Accordingly, in the second regions 10 t in theIII-nitride crystal 10, the polarity in the <0001> directions isinverted with respect to the first region 10 s, and the dislocationdensity is raised.

The crystal-growth rate in the first region 10 s, however, is greaterthan that in the second regions 10 t. Therefore, along with the growthof the III-nitride crystal 10, the first region 10 s covers over andburies the second regions 10 t. Growing the III-nitride crystal 10 inthis manner leads to crystal in which, with the crystal being grownbeyond a certain thickness, the first region 10 s exists alone, makingit possible to manufacture III-nitride crystal at least the surface ofwhich has a single polarity, with dislocation density being low acrossthe surface globally. That is, in this embodiment mode, giving attentionto the fact that the crystal-growth rate in the first region 10 s isgreater than that in the second regions 10 t enables manufacturingIII-nitride crystal at least the surface of which has a single polaritywith dislocation-density being low across the surface globally, without,in an undersubstrate as set forth in Patent Document 4, deep-etchingremoval of the entirety or a portion of the domains of one polarity.

As just described, the inventors of the present invention discovered anew phenomenon in which, in III-nitride crystal growth by a liquid-phasetechnique, a great difference arises between the growth rate at whichcrystal grows onto inversion domains in seed crystal, and the growthrate at which crystal grows onto the matrix therein. One feature of thepresent invention is to exploit this discovery to afford technologywhereby crystal of, at least in the surface, single polarity globallyand low dislocation density globally is manufactured by preparing seedcrystal having inversion domains where defect density is regionallyhigh, and burying the inversion domains.

Herein, in the III-nitride crystal growth, the crystal thickness atwhich the second regions 10 t become covered with the first region 10 s(also referred to as the “second region-covering crystal thickness,”likewise hereinafter) is determined by the area of the surface of theinversion domains 1 t, and on the crystal-growth conditions for theIII-nitride crystal 10.

In Embodiment Mode 1, while the liquid-phase technique for growingIII-nitride crystal 10 onto an undersubstrate 1 is not particularlylimited, inasmuch as creating the high-temperature, high-pressureconditions whereby III-nitride crystal becomes molten is problematicwith facilities for manufacturing practical 2-inch diameter crystal,solution deposition is preferably utilized. Furthermore, from ajob-safety perspective, it is particularly preferable to utilize chieflya Group III element-containing melt for the solution.

In Embodiment Mode 1, the layout of the matrix 1 s and inversion domains1 t in the III-nitride seed crystal contained in the undersubstrate 1 isnot particularly limited, but from the perspective of allowing the firstregion 10 s to efficiently cover the second regions 10 t in the growthof III-nitride crystal 10, it is preferable to have the layout be as thefollowing embodiment modes.

Embodiment Mode 1A

Referring to FIGS. 3 through 5, in one embodiment mode of the layout ofthe inversion domains 1 t in the undersubstrate 1, the inversion domains1 t are in the form of a plurality of stripe domains 1 t _(a) and 1 t_(b) along a {0001} plane on the undersubstrate 1, with the stripeddomains 1 t _(a) and 1 t _(b) each being arranged parallel to each otherat regular intervals. From the perspective of carrying out uniformcrystal growth, preferable is that the striped domains 1 t _(a) and 1 t_(b) each have the predetermined width of W, and are arranged parallelto each other with the predetermined pitch of P. Furthermore, a stripeorientation of each of the striped domains 1 t _(a) and 1 t _(b) is notparticularly limited, but from the perspective of stably growinginversion domains, the stripe orientation preferably parallels the<1-100> or <11-20> directions. That is because the fact that in liquiddeposition, growth rate is slower compared with that in HVPE and otherhigh-productivity vapor-phase techniques causes crystal habitsreflecting crystallographic structure to tend to appear, meaning that(not-illustrated) hexagonal columnar or hexagonal trapezoidal terracesare liable to form on the surface on whish crystal is grown, and crystalgrowth of the lateral sides of the terraces mainly in the orientationperpendicular to the <0001> directions, as well as in the orientationperpendicular to the <1-100> directions causes the first region 10 s tocoalesce at the sides paralleling each other, which prompts the growthof the first region 10 s, in the growth of the III-nitride crystal 10.

An example of the stripe orientation, paralleling the <1-100>directions, of each of striped domains is represented in FIG. 3, anexample of the stripe orientation paralleling the <11-20> directions isrepresented in FIG. 4, and an example of the stripe orientationparalleling the direction rotated 45° in the <11-20> directions withrespect to the <1-100> directions is represented in FIG. 5.

Embodiment Mode 1B

Referring to FIGS. 6 and 7, in still another embodiment mode of thelayout of the inversion domains 1 t in the undersubstrate 1, theinversion domains 1 t are in the form of a plurality of dot domains 1 t_(m) and 1 t _(n) along a {0001} plane on undersubstrate, with thedotted domains 1 t _(m) and 1 t _(n) each being arrangedtwo-dimensionally at regular intervals. Arranging the inversion domains1 t as the dotted domains 1 t _(m) and 1 t _(n) enables enlarging thearea of the surface of the matrix 1 s, compared with arranging theinversion domains 1 t as striped domains (Embodiment Mode 1A), meaningthat the area of the surface of a first region 10 s(low-dislocation-density region) in III-nitride crystal to be grownincreases, so that second regions (high-dislocation-density regions) canbe covered even if the III-nitride crystal thickness is smaller.

Herein, the two-dimensional, regular layout is not particularly limited.In FIG. 6, the dotted domains 1 t _(m) and 1 t _(n) with a diameter of Ware arranged so that their centers are positioned respectively at thevertices of close-packed squares 1 f, arranged two-dimensionally, havinga side of P. Herein, the orientation of either pair of facing sides ofthe squares 1 f parallels the <1-100> directions. In other words, thedotted domains 1 t _(m) and 1 t _(n) whose diameter is W are eacharranged paralleling the <1-100> and <11-20> directions with a constantpitch of P.

Moreover, as illustrated in FIG. 7, from the perspective of readilyconcentrating dislocations into inversion domains, the dotted domains 1t _(m) and 1 t _(n) with a diameter of W are preferably arranged so thattheir centers are positioned respectively at the vertices ofclose-packed regular triangles 1 g, arranged two-dimensionally, having aside of P. Herein, the orientation of any one side of each of the aboveregular triangles 1 g preferably in particular parallels the <1-100>directions. That is because in liquid deposition, (not-illustrated)hexagonal columnar or hexagonal trapezoidal terraces tend to form on thesurface on which crystal is grown, and the fact that the lateral sidesof the terraces grow mainly in the orientation perpendicular to the<0001> directions, as well as in the orientation perpendicular to the<1-100> directions causes the first region 10 s to coalesce at the sidesparalleling each other, which prompts the growth of the first region 10s, in the growth of the III-nitride crystal 10.

Embodiment Mode 1C

Referring to FIGS. 8 and 9, in a further embodiment mode of the layoutof the inversion domains 1 t in the undersubstrate, the inversiondomains 1 t are in a honeycomb form, arranged in close-packed regularhexagons 1 h two-dimensionally along a {0001} plane on theundersubstrate 1, having sides L. Herein, “honeycomb” means the regionhaving width W, centered on the six sides of the regular hexagons 1 h,as illustrated in FIGS. 8 and 9.

Herein, the orientation of the above six sides is not particularlylimited, but from the perspective of stably growing inversion domains,preferably parallels the <1-100> or <11-20> directions. III-nitridecrystal having a wurtzite crystallographic structure, the orientation ofthe above six sides illustrated in FIG. 8 parallels the <11-20>directions, and the orientation of the above six sides illustrated inFIG. 9 parallels the <1-100> directions, since the crystallographicstructure is of six-fold symmetry in a {0001} plane. Moreover, it isparticularly preferable that the orientation of the six sides parallelsthe <1-100> directions. This is because in liquid deposition,(not-illustrated) hexagonal columnar or hexagonal trapezoidal terracesreadily form on the crystal-growth surface, and since the lateral sidesof the terraces grow in directions perpendicular to the <0001>directions, as well as in directions perpendicular to the <1-100>directions, first regions 10 s in the growth of the III-nitride crystal10 unite along surfaces paralleling each other, thanks to which growthof the first region 10 s is promoted.

Furthermore, in Embodiment Mode 1, III-nitride crystal is preferablygrown to a thickness of 1 μm or more. Because growing a III nitridecrystal 10 having a thickness of 1 μm or more onto the matrix 1 s andinversion domains 1 t of the undersubstrate 1 by a liquid-phasetechnique ensures that the first region 10 s covers the second regions10 t, III-nitride crystal at least the surface of which has a singlepolarity, with dislocation density being low across the surface globallycan be readily manufactured.

Moreover, in III-nitride crystal, the surface preferably has aresistivity of 1×10⁵ Ω·cm or more, from the perspective of forminghigh-resistivity substrates employed in, for example, HEMTs. Herein, theIII-nitride crystal whose surface has a resistivity of 1×10⁵ Ω·cm ormore can be readily manufactured by the above manufacturing method.

Also, in Embodiment Mode 1, a reaction vessel for growing III-nitridecrystal is not particularly limited as long as it is chemically stablewith sufficient heat-resistance, and is high in mechanical strength, andthe reaction vessel is preferably formed of oxides such as aluminumoxide (Al₂O₃), or of nitrides such as pyrolytic boron nitride (pBN).From the perspective of preventing impurities from being mixed intocrystal, and of enabling manufacturing more readilylow-dislocation-density, high-resistivity crystal, the reaction vesselis more preferably formed of nitrides such as pBN.

Embodiment Mode 2

Referring to FIG. 2, another embodiment mode of a III-nitride crystalmanufacturing method involving the present invention includes: a step(FIG. 2A) of preparing an undersubstrate 1 containing a III-nitride seedcrystal, the III-nitride seed crystal having a matrix 1 s and inversiondomains 1 t in which the polarity in the <0001> directions is invertedwith respect to the matrix 1 s; and a step (FIG. 2B) of growing aIII-nitride crystal 10 onto the matrix 1 s and inversion domains 1 t ofthe undersubstrate 1 by a liquid-phase technique. Herein, thisembodiment mode is characterized in that a first region 10 s, beingwhere the growth rate of the III-nitride crystal 10 growing onto thematrix is greater, covers second regions 10 t, being where the growthrate of the III-nitride crystal 10 growing onto the inversion domains 1t is lesser. Also, the embodiment mode is characterized in that in theundersubstrate 1, the surface of the inversion domains 1 t is recessedrelative to the surface of the matrix 1 s.

That is, in this embodiment mode, referring to FIG. 2, in theundersubstrate 1 for growing the III-nitride crystal 10 in EmbodimentMode 1, the surface of the inversion domains 1 t is recessed relative tothe surface of the matrix 1 s. More precisely, the undersubstrate 1 hasthe surface with recesses and terraces, and the surface of the inversiondomains 1 t forms the recesses, while the surface of the matrix 1 sforms the terraces.

In this embodiment mode, the fact that the surface of the inversiondomains 1 t of, is recessed relative to the surface of the matrix 1 sof, the undersubstrate 1 gives higher priority to the growth of thefirst region 10 s growing onto the matrix 1 s over the growth of thesecond regions 10 t growing onto the inversion domains 1 t in growingthe III-nitride crystal 10 onto the surface of the undersubstrate 1,compared with the case in Embodiment Mode 1 to reduce secondregion-covering crystal thickness. Therefore, growth of thinnercrystal—that is, crystal growth for shorter period of time—makes itpossible to manufacture III-nitride crystal at least the surface ofwhich has a single polarity, and the dislocation density is lowglobally.

Also in Embodiment Mode 2, as in Embodiment Mode 1, preferable is that:the layout of the inversion domains 1 t in undersubstrate is the samelayout as any of Embodiment Modes 1A, 1B, and 1C; III-nitride crystal isgrown to a thickness of 1 μm or more; in the III-nitride crystal, thesurface has a resistivity of 1×10⁵ Ω·cm or more; a reaction vessel forgrowing the III-nitride crystal is formed of oxides such as Al₂O₃ ornitrides such as pBN, with nitrides such as pBN being more preferable.

Embodiment Mode 3

Referring to FIGS. 1, 2, and 11, a further embodiment mode of aIII-nitride crystal manufacturing method involving the present inventionincludes: a step (FIGS. 1A and 2A) of preparing an undersubstrate 1containing a III-nitride seed crystal, the III-nitride seed crystalhaving a matrix 1 s and inversion domains 1 t in which the polarity inthe <0001> directions is inverted with respect to the matrix 1 s; a step(FIGS. 1B and 2B) of growing a III-nitride crystal 10 onto the matrix 1s and inversion domains 1 t of the undersubstrate 1 by a liquid-phasetechnique; and a step (FIGS. 11A and 11B) of further growing by avapor-phase technique the III-nitride crystal 10 grown onto theundersubstrate 1 by the liquid-phase technique.

That is, Embodiment Mode 3 includes the step (FIGS. 11A and 11B) offurther growing by a vapor-phase technique the III-nitride crystal 10that has been grown onto the undersubstrate 1 by a liquid-phasetechnique in Embodiment Mode 1 or 2. Such a step enables manufacturingefficiently, at a high growth rate and at low cost, III-nitride crystalat least whose surface has—with dislocation density being low globally—asingle polarity. Herein, as illustrated in FIG. 11B, in the III-nitridecrystal 10, a first region 10 s and second regions 10 t grown by aliquid-phase technique unites with a vapor-deposited region 10 v grownby a vapor-phase technique.

Herein, the vapor-phase technique is not particularly limited, but fromthe perspective of facilitating epitaxial growth, HVPE, metalorganicchemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE) arepreferable, for example. Furthermore, from the perspective of highcrystal-growth rate, HVPE is particularly preferable.

Embodiment Mode 4

Referring to FIGS. 10A, 10B, 11B, and 11C, one embodiment mode of aIII-nitride crystal substrate involving the present invention isIII-nitride crystal substrates L1, L2, V1, V2, V3, V4, V5, V6, and V7produced by processing a III-nitride crystal 10 manufactured by any ofEmbodiment Modes 1 through 3. Herein, the III-nitride crystal substratesL1 and L2 represent the substrates produced from the regions (firstregion 10 s and second regions 10 t) grown by a liquid-phase technique,and the III-nitride crystal substrates V1, V2, V3, V4, V5, V6, and V7represent the substrates produced from the region (vapor-depositedregion 10 v) grown by a vapor-phase technique. Additionally, in theIII-nitride crystal substrate L1, the III-nitride seed crystal servingas undersubstrate can be included.

The III-nitride crystal substrates produced in this manner are ideallysuited to utilization as substrates for semiconductor devices, becauseat least the surface has—with dislocation density being low globally—asingle polarity.

The III-nitride crystal processing method is not particularly limited,and may include, for example, a step of slicing or cleaving III-nitridecrystal parallel to its principal face into the form of a plate. Thestep of slicing or cleaving makes it possible to readily produceplate-shaped III-nitride crystal substrates. Moreover, the III-nitridecrystal processing method may include a step of grinding and/orpolishing the principal face of the produced III-nitride crystalsubstrates. Also, a step of removing by reactive ion etching (RIE) orother vapor-phase etching a damaged layer on the principal face afterthe grinding and/or polishing may be further included. Onto theprincipal face of the III-nitride crystal substrates whose principalfaces have been ground and/or polished, an at least single-lamina IIInitride semiconductor layer having preferable crystallinity may beformed.

Embodiment Mode 5

Referring to FIGS. 12 and 13, one embodiment mode of a III-nitridesemiconductor device involving the present invention is a III-nitridesemiconductor device in which an at least single-lamina III nitridesemiconductor layer 110 or 120 is formed onto the III-nitride crystalsubstrate 100 of Embodiment Mode 4. The III-nitride semiconductor devicehas a III-nitride crystal substrate at least whose surface has—withdislocation density being low globally—a single polarity. Therefore, aIII-nitride semiconductor layer formed onto this III-nitride crystalsubstrate has low dislocation density, satisfactory crystallinity, andenhanced properties. Examples of the III-nitride semiconductor device, aLD and HEMT are given below.

Embodiment Mode 5A

In an LD that is one example of the III-nitride semiconductor deviceinvolving the present invention, referring to FIG. 12, as the at leastsingle-lamina III-nitride semiconductor layer 110, the following layersare formed successively onto a first principal face of the III-nitridecrystal substrate 100 at least whose surface has—with dislocationdensity being low globally—a single polarity: an n-typeAl_(0.1)Ga_(0.9)N cladding layer 111; an n-type GaN guide layer 112; amultiquantum well active layer 113 (emission layer) composed of fourpairs of InGaN/GaN layers; a p-type Al_(0.2)Ga_(0.8)N protective layer114; a p-type GaN guide layer 115; a p-type Al_(0.1)Ga_(0.9)N claddinglayer 116; and a p-type GaN contact layer 117. Onto the p-type GaNcontact layer 117, a PdAu alloy electrode is formed as a p-sideelectrode 118. Herein, mesa-etching through the p-side electrode 118,the p-type GaN contact layer 117, and to a depth into the p-typeAl_(0.1)Ga_(0.9)N cladding layer 116 is carried out to form a ridgeportion. Additionally, a Ti/Al alloy electrode is formed as an n-sideelectrode 119 onto a second principal face of the III-nitride crystalsubstrate 100.

Embodiment Mode 5B

In the HEMT that is another example of the III-nitride semiconductordevice involving the present invention, referring to FIG. 13, as the atleast single-lamina III-nitride semiconductor layer 120, the followinglayers are formed successively onto a first principal face of aIII-nitride crystal substrate 100 at least whose surface has—withdislocation density being low globally—a single polarity: an undopedAl_(0.26)Ga_(0.74)N spacer layer 121; a Si-doped n-typeAl_(0.26)Ga_(0.74)N carrier supply layer 122; and a Si-doped n-type GaNcontact layer 123. Onto the Si-doped n-type GaN contact layer 123, Ti/Alserving as a source electrode 126 and a drain electrode 127 is formed byelectron beam evaporation, and is alloyed by heat treatment, to formTi/Al alloy electrodes. Furthermore, a central portion of the Ti/Alalloy electrodes, of the Si-doped n-type GaN contact layer 123, and of apart of the Si-doped n-type Al_(0.26)Ga_(0.74)N carrier supply layer 122is removed by recess-etching into the form of a stripe, and onto theexposed Si-doped n-type Al_(0.26)Ga_(0.74)N carrier supply layer 122, aPt/Ti/Au layer-laminated electrode is formed as a gate electrode 125 byelectron beam evaporation so as not to contact with the source electrode126 and drain electrode 127. Herein, in the gate electrode 125, thePt/Ti/Au layers are formed successively from the side of the Si-dopedn-type Al_(0.26)Ga_(0.74)N carrier supply layer 122.

EMBODIMENTS Embodiment 1 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 3, a GaN seed crystal that was 2 inches (50.8mm) in diameter and 350 μm in thickness, and that had a matrix 1 s witha dislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Furthermore, the inversion domains 1 t were in theform of a plurality of stripe domains 1 t _(a) and 1 t _(b) along the(0001) plane on the undersubstrate 1, with the striped domains 1 t _(a)and 1 t _(b) each having width W of 50 μm, and being arranged parallelto each other with pitch P of 300 μm. Moreover, the stripe orientationof each of the striped domains 1 t _(a) and 1 t _(b) paralleled the<1-100> directions.

Additionally, the undersubstrate 1 in Embodiment 1 was produced by,based on the growth method set forth in Patent Document 1, growing GaNcrystal by HVPE onto a GaAs substrate in which mask layers in the formof a plurality of stripes were formed onto the (111) A-face. Herein, thestriped mask layers each had width of 50 μm, and were arranged parallelto each other with pitch of 300 μm. Moreover, the stripe orientation ofeach of the striped mask layers paralleled the <11-2> directions in theGaAs substrate. That is, the inversion domains 1 t of the GaN crystalwere formed onto the striped mask layers in the GaAs substrate, and thematrix 1 s was formed onto the GaAs substrate except where the masklayers were formed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal, or a III-nitride crystal 10 was grown onto the aboveundersubstrate 1 by solution deposition. Specifically, in an Al₂O₃crucible (reaction vessel 7) that was 52 mm in inner diametric span and30 mm in height, with the undersubstrate 1 being placed on the cruciblebottom, metal Ga of 12 g was put, and heated to form a Ga melt(crystal-growth solution 2) on the undersubstrate 1. Next, temperaturesof the undersubstrate 1 and Ga melt (crystal-growth solution 2) werebrought to 950° C., and a N₂ gas was supplied as a nitrogen-containinggas 3 to the Ga melt (crystal-growth solution 2) to bring the N₂ gaspressure to 6.078 MPa (60 atmospheres), to grow the GaN crystal(III-nitride crystal 10) for 100 hours. After being cooled, residual Gain the crucible was removed with hydrochloric acid, and then the GaNcrystal (III-nitride crystal 10) growing onto the undersubstrate 1 tounite with it was picked out. The GaN crystal thickness was 3 μm.

(3)III Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with the undersubstrate1 was observed for polarity and dislocation density in the followingmanner.

The GaN crystal uniting with undersubstrate was immersed in aphosphoric-sulfuric acid mixed solution, heated to 250° C. to etch itfor one hour, and observed under an optical microscope. The observationresult proved that the portions grown onto the inversion domains 1 t ofthe undersubstrate 1 also had a tendency to stand etching, as the otherportion. Furthermore, observing the GaN crystal uniting withundersubstrate under a fluorescence microscope revealed that any zonelooking like reflecting an inversion of the GaN crystal polarity—thatis, any zone in which emission was changed was not found. These resultsconfirmed that as to the surface of the GaN crystal uniting withundersubstrate, the entire surface was made the Ga face (the (0001)plane)—that is, the entire surface had a single polarity.

Moreover, as a result of observation by cathodoluminescence, the surfacedislocation density in the GaN crystal uniting with undersubstrate was alow 1×10⁴ to 1×10⁵ cm⁻² uniformly across the entire surface.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 1 was crystal, in GaNcrystal of 3 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

(4) LD Fabrication

Referring to FIG. 12, GaN crystal uniting with such an undersubstratewas utilized as a III-nitride crystal substrate 100, and an at leastsingle-lamina III-nitride semiconductor layer 110 was formed onto theIII-nitride crystal substrate 100, to fabricate LDs, next.

Specifically, the III-nitride crystal substrate 100 was polished, and adamaged layer on the substrate surface was removed by reactive ionetching (RIE). Subsequently, as the III-nitride semiconductor layer 110,the following layers were formed successively onto a first principalface of the III-nitride crystal substrate 100: an 3 μm-thick n-typeAl_(0.1)Ga_(0.9)N cladding layer 111; a 0.1 μm-thick n-type GaN guidelayer 112; a 0.05 μm-thick multiquantum well active layer 113 (emissionlayer) composed of four pairs of InGaN/GaN layers; a 0.02 μm-thickp-type Al_(0.2)Ga_(0.8)N protective layer 114; a 0.1 μm-thick p-type GaNguide layer 115; a 0.4 μm-thick p-type Al_(0.1)Ga_(0.9)N cladding layer116; and a 0.1 μm-thick p-type GaN contact layer 117. The formation ofthe III-nitride semiconductor layer 110 was carried out by MOCVD withsubstrate temperature being made 1080° C., utilizing: a trimethylgallium and trimethylaluminum gases as III element source gas; anammonia gas as nitrogen source gas; a hydrogen gas as carrier gas; asilane gas as n-type dopant gas; and magnesium as a p-type dopant.

Next, that parts of the p-type GaN contact layer 117 which were formedon both edges and a part of the p-type Al_(0.1)Ga_(0.9)N cladding layer116 were removed by mesa-etching to form a ridge with ridge stripe widthof 2 μm. Subsequently, a Pd/Au alloy electrode was formed as a p-sideelectrode 118 onto the p-type GaN contact layer 117 by electron beamevaporation and heat treatment.

Next, a second principal face of the III-nitride crystal substrate 100was polished to bring the substrate thickness to 100 μm, and then aTi/Al alloy electrode was formed as an n-side electrode 119 onto thesecond principal face by electron beam evaporation and heat treatment.

After that, the wafer in which the III-nitride semiconductor layer 110and p-side electrode 118 were formed onto the first principal face ofthe III-nitride crystal substrate 100 and the n-side electrode 119 wasformed onto the second principal face was divided into square chipshaving of 300 μm to fabricate LDs. From the wafer that was 2 inches(50.8 mm) in diameter, 1,000 LDs were fabricated. These LDs were theblue-violet LDs having threshold current of 45 mA and oscillationwavelength of 405 nm. In a life test on these LDs—in the test, the LDswere oscillated on power of 30 mW under an atmosphere having temperatureof 60° C.—qualified LDs whose lifetimes were estimated to be 10,000hours or more were obtained at yield of 80%.

Comparative Example 1

The undersubstrate 1 employed in Embodiment 1 (that is, the GaN seedcrystal that is 2 inches (50.8 mm) in diameter and 350 μm in thickness,having the matrix 1 s with a dislocation density of 1×10⁴ to 1×10⁵ cm⁻²and the inversion domains 1 t with a dislocation density of 1×10⁸ to1×10⁹ cm⁻²) was utilized as a III-nitride crystal substrate 100 tofabricate LDs in the same manner as in Embodiment 1. In the LDfabrication, it was made sure that a ridge portion was formed in aregion right above the matrix 1 s.

From the wafer that was 2 inches (50.8 mm) in diameter, 1,000blue-violet LDs having oscillation wavelength of 405 nm were fabricated.As a result of carrying out the same life test as in Embodiment 1 onthese LDs, qualified-LD yield was 10%. Analyzing broken-down LDs provedthat in most of the parts formed onto inversion domains 1 t in aIII-nitride semiconductor layer formed onto the undersubstrate 1, thepolarity was inverted. Furthermore, in the part formed onto a matrix 1 sin the III-nitride semiconductor layer formed on the undersubstrate 1,unusual growth—for example, the edges of the part rose—occurred.

Comparative Example 2 (1) Undersubstrate Preparation

A GaN crystal substrate fabricated by ELO of Patent Document 2 wasprepared as an undersubstrate. Specifically, a mask composed of SiO₂film was formed by chemical vapor deposition (CVD) onto a substrate inwhich a 1 μm-thick GaN layer was formed by MOCVD onto the (0001) planeon a sapphire substrate that was 2 inches (50.8 mm) in diameter and 300μm in thickness to pattern the mask into the form of stripes byphotolithography and wet-etching. The left striped masks each had maskwidth of 5 μm, and were arranged with pitch of 7 μm, with the stripeorientation being in the <11-20> directions in the GaN crystal.

Next, the substrate with the striped masks was inserted into a HVPEapparatus, and with temperature being raised to 1,000° C. under a H₂ gasatmosphere, a GaCl and NH₃ gases were supplied for 25 minutesrespectively at 20 sccm (hereinafter, 1 sccm is 1,013 hPa, which meansthe flow rate at which a gas at 0° C. in standard condition flows 1 cm³for one minute) and at 1,000 sccm to laterally grow GaN crystal growingthrough the openings of the striped masks. Furthermore, the GaN crystalwas grown for 10 hours to obtain 400 μm-thick GaN crystal in which theentire surface was planar.

Subsequently, after being cooled, the GaN crystal was picked out fromthe HVPE apparatus, and third harmonics (wavelength of 355 nm) ofyttrium aluminum garnet laser (YAG laser) were radiated at the entiresurface from the sapphire substrate side to decompose a GaN layer lyingat the interface between the GaN crystal and the sapphire substrate,separating the GaN crystal from the sapphire substrate. Successively,the both faces of the GaN crystal were polished to produce 360 μm-thickGaN crystal.

The produced GaN crystal was immersed in a phosphoric-sulfuric acidmixed solution, heated to 250° C. to etch it for one hour, and observedunder an optical microscope. As a result, the entire surface of the GaNcrystal was proved to be the Ga face ((0001) plane). Furthermore,observation by CL revealed that the surface dislocation density in theGaN crystal was 1×10⁷ to 1×10⁸ cm⁻². The surface of the above GaNcrystal was polished to produce a 350 μm-thick GaN crystal substrate.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

A GaN crystal (III-nitride crystal 10) was grown onto the above GaNcrystal substrate (undersubstrate 1) by solution deposition for 100hours under the same conditions as in Embodiment 1, and the GaN crystal(III-nitride crystal 10) united with the undersubstrate 1 in the samemanner as in Embodiment 1 was picked out. The GaN crystal thickness was3 μm.

(3) III-Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a high 1×10⁷ to 1×10⁸ cm⁻².

(4) LD Fabrication

The GaN crystal uniting with undersubstrate was utilized as aIII-nitride crystal substrate 100 to fabricate 1,000 LDs in the samemanner as in Embodiment 1. The lifetime of the LDs, however, could notbe measured because all of the fabricated LDs did not oscillate on powerof 30 mW even under a room temperature atmosphere (for example, 25° C.).Therefore, qualified-LD yield was 0%.

Embodiment 2 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 4, GaN seed crystal that was 2 inches (50.8 mm)in diameter and 350 μm in thickness, having a matrix 1 s with adislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Furthermore, the inversion domains 1 t were in theform of a plurality stripe domains 1 t _(a) and 1 t _(b) along the(0001) plane on the undersubstrate 1, with the striped domains 1 t _(a)and 1 t _(b) each having width W of 50 μm, and being arranged parallelto each other with pitch P of 300 μm. Furthermore, the stripeorientation of each of the striped domains 1 t _(a) and 1 t _(b)paralleled the <11-20> directions.

Additionally, the undersubstrate in Embodiment 2 was produced by, basedon the growth method set forth in Patent Document 1, growing GaN crystalby HVPE onto a GaAs substrate in which mask layers in the form of aplurality of stripes were formed on the (111) A-face. Herein, thestriped mask layers each had thickness of 50 μm, and were arrangedparallel to each other with pitch of 300 μm. Furthermore, the stripeorientation of each of the striped mask layers paralleled the <−110>directions in the GaAs substrate. That is, the inversion domains 1 t ofthe GaN crystal were formed onto the striped mask layers in the GaAssubstrate, and the matrix 1 s was formed onto the GaAs substrate exceptwhere the mask layers were formed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal (III-nitride crystal 10) was grown onto the aboveundersubstrate 1 by solution deposition for 100 hours under the sameconditions as in Embodiment 1, and the GaN crystal (III-nitride crystal10) united with the undersubstrate 1 in the same manner as in Embodiment1 was picked out. The GaN crystal thickness was 3 μm.

(3) III-Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a low 1×10⁴ to 1×10⁵ cm⁻² uniformly across the crystal surfaceglobally.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 2 was crystal, in GaNcrystal of 3 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

Embodiment 3 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 5, GaN seed crystal that was 2 inches (50.8 mm)in diameter and 350 μm in thickness, having a matrix 1 s with adislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Furthermore, the inversion domains 1 t were in theform of a plurality of stripe domains 1 t _(a) and 1 t _(b) along the(0001) plane on the undersubstrate 1, with the striped domains 1 t _(a)and 1 t _(b) each having width W of 50 μm, and being arranged parallelto each other with pitch P of 300 μm. Moreover, the stripe orientationof each of the striped domains 1 t _(a) and 1 t _(b) paralleled thedirection rotated 45° in the <11-20> directions with respect to the<1-100> directions.

Additionally, the undersubstrate in Embodiment 3 was produced by, basedon the growth method set forth in Patent Document 1, growing GaN crystalby HVPE onto a GaAs substrate in which mask layers in the form of aplurality of stripes were formed on the (111) A-face. Herein, thestriped mask layers each had width of 50 μm, and were arranged parallelto each other with pitch of 300 μm. Moreover, the stripe orientation ofeach of the striped mask layers paralleled the direction rotated 45° inthe <−100> directions with respect to the <11-2> directions in the GaAssubstrate. That is, the inversion domains it of the GaN crystal wereformed onto the striped mask layers of the GaAs substrate, and matrix 1s was formed onto the GaAs substrate except where the mask layers wereformed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal (III-nitride crystal 10) was grown onto the aboveundersubstrate 1 by solution deposition for 200 hours under the sameconditions as in Embodiment 1, and the GaN crystal (III-nitride crystal10) united with the undersubstrate 1 in the same manner as in Embodiment1 was picked out. The GaN crystal thickness was 6 μm.

(3)III Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a low 1×10⁴ to 1×10⁵ cm⁻² uniformly across the crystal surfaceglobally.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 3 was crystal, in GaNcrystal of 6 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

Embodiment 4 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 5, the same GaN seed crystal as that inEmbodiment 3 was prepared as an undersubstrate 1. Subsequently, thisundersubstrate was immersed in a molten KOH solution, and was heated at300° C. for 10 minutes. Herein, because the speed at which N face wasetched with molten KOH is remarkably faster compared with the speed atwhich Ga face was etched with molten KOH, the undersubstrate 1 in whichthe surface of the inversion domains 1 t was recessed by 20 μm relativeto the surface of the matrix 1 s, as illustrated in FIGS. 2 and 5, wasproduced.

(2) III-Nitride Crystal Growth

GaN crystal (III-nitride crystal 10) was grown onto the undersubstratehaving the rough surface with the recesses of the inversion domains 1 t,by solution deposition for 36 hours under the same conditions as inEmbodiment 1, and the GaN crystal (III-nitride crystal 10) united withthe undersubstrate 1 in the same manner as in Embodiment 1 was pickedout. The GaN crystal thickness was 1 μm.

(3) III-Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a low 1×10⁴ to 1×10⁵ cm⁻² uniformly across the crystal surfaceglobally.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 4 was crystal, in GaNcrystal of 1 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

Comparing Embodiment 4 with Embodiment 3 verified that recessing theinversion domains 1 t of the undersubstrate 1 to grow the III-nitridecrystal 10 gave higher priority to the growth of the first region 10 sgrowing onto the matrix is over the growth of the second regions 10 tgrowing onto the inversion domains 1 t, decreasing the secondregion-covering crystal thickness, so that growth of thinnercrystal—that is, crystal growth for shorter period of time—made itpossible to produce III-nitride crystal having a single polarity, withthe dislocation density at least in the surface being low globally.

Embodiment 5 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 6, GaN seed crystal that was 2 inches (50.8 mm)in diameter and 350 μm in thickness, having a matrix 1 s with adislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Herein, the inversion domains 1 t were in the form ofa plurality of dot domains 1 t _(m) and 1 t _(n) along the (0001) planeon the undersubstrate 1, with the dotted domains 1 t _(m) and 1 t _(n)having diameter W of 50 μm each being arranged so that their centerswere positioned respectively at the vertices of close-packed squares 1f, arranged two-dimensionally, having sides P of 300 μm. Furthermore,the orientation of either pair of facing sides of the squares 1 fparalleled the <1-100> directions.

Additionally, the undersubstrate in Embodiment 5 was produced by, basedon the growth method set forth in Patent Document 1, growing GaN crystalby HVPE onto a GaAs substrate in which mask layers in the form of aplurality of dots were formed on the (111) A-face. Herein, the dottedmask layers each had diameter of 50 μm, and were arranged so that theircenters were positioned respectively at the vertices of close-packedsquares, arranged two-dimensionally, having sides P of 300 μm. Moreover,the orientation of either pair of facing sides of the squares 1 fparalleled the <11-2> directions in the GaAs substrate. That is, theinversion domains 1 t of the GaN crystal were formed onto the masklayers of the GaAs substrate, and matrix 1 s was formed onto the GaAssubstrate except where the mask layers were formed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal (III-nitride crystal 10) was grown onto the aboveundersubstrate 1 by solution deposition for 100 hours under the sameconditions as in Embodiment 1, and the GaN crystal (III-nitride crystal10) united with the undersubstrate 1 in the same manner as in Embodiment1 was picked out. The GaN crystal thickness was 3 μm.

(3) III-Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a low 1×10⁴ to 1×10⁵ cm⁻² uniformly across the crystal surfaceglobally.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 5 was crystal, in GaNcrystal of 3 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

Embodiment 6 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 7, GaN seed crystal that was 2 inches (50.8 mm)in diameter and 350 μm in thickness, having a matrix 1 s with adislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Herein, the inversion domains 1 t were in the form ofa plurality of dot domains 1 t _(m) and 1 t _(n) along the (0001) plane,with the dotted domains 1 t _(m) and 1 t _(n) having diameter W of 50 μmeach being arranged so that their centers were positioned respectivelyat the vertices of close-packed regular triangles 1 g, arrangedtwo-dimensionally, having sides P of 300 μm. Furthermore, theorientation of all three sides of the regular triangles 1 g paralleledthe <1-100> directions.

Additionally, the undersubstrate in Embodiment 6 was produced by, basedon the growth method set forth in Patent Document 1, growing GaN crystalby HVPE onto a GaAs substrate in which mask layers in the form of aplurality of dots were formed on the (111) A-face. Herein, the dottedmask layers each had diameter of 50 μm, and were arranged so that theircenters were positioned respectively at the vertices of close-packedregular triangles, arranged two-dimensionally, having sides P of 300 μm.Moreover, the orientation of any one side of each of the regulartriangles paralleled the <11-2> directions in the GaAs substrate. Thatis, the inversion domains 1 t of the GaN crystal were formed onto thedotted mask layers on the GaAs substrates, and matrix 1 s was formedonto the GaAs substrate except where the mask layers were formed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal was grown onto the above undersubstrate 1 by solutiondeposition under temperature and pressure conditions higher than inEmbodiment 1, utilizing as material for the reaction vessel 7 pBN thatwas high-purity material for crucible. Specifically, with growthtemperature being brought to 1,500° C., and with N₂ gas pressure beingbrought to 1.5 GPa (approximately 15,000 atmospheres), the GaN crystal(III-nitride crystal 10) was grown for 200 hours, and the GaN crystal(III-nitride crystal 10) united with the undersubstrate 1 in the samemanner as in Embodiment 1 was picked out. The GaN crystal thickness was650 μm.

(3) III-Nitride Crystal Evaluation

Referring to FIG. 10, the 1,000 μm-thick GaN crystal (III nitridecrystal 10) united with the undersubstrate 1 was sliced with a slicerinto two slices, which were each polished along the side grown bysolution deposition, and furthermore the surface-damaged layer on theGa-face side was removed with RIE. As a result, one GaN crystalsubstrate (III-nitride crystal substrate L1) in which the 350 μm-thickundersubstrate having the matrix 1 s and inversion domains 1 t andsolution-grown 100 μm-thick GaN crystal united together, and one 450μm-thick GaN crystal substrate (III-nitride crystal substrate L2)composed of the region grown by solution deposition were produced.

Next, four-terminal Hall measurement was carried out on the Ga facesides of the substrates L1 and L2. The measurement results confirmedthat the substrates both were high-resistivity substrates having aresistivity of 1×10⁵ Ω·cm or more—that is, they both are substratessuitable for HEMT and other devices.

Furthermore, the substrates L1 and L2 were observed under an optical andfluorescence microscopes after the same etching as in Embodiment 1,which proved that the entirety of the substrate surfaces were the Gaface (the (0001) plane), and had a single polarity. Moreover,observation by CL revealed that dislocation density in the substrates L1and L2 (III-nitride crystal 10) was a low 1×10⁴ to 1×10⁵ cm⁻² uniformlyacross the substrate surfaces globally.

From the above-noted results it could be confirmed that the GaN crystalsubstrates (substrates L1 and L2) of Embodiment 6 were crystal in whichat least the surface had a single polarity, and the dislocation densitywas low globally.

(4) HEMT Fabrication

Referring to FIG. 13, such high-resistivity GaN crystal substrates wereutilized as III-nitride crystal substrates 100, and an at leastsingle-lamina III-nitride semiconductor layer 120 was formed onto theIII-nitride crystal substrates 100, to fabricate HEMTs.

Specifically, the III-nitride crystal substrates 100 were polished, anda damaged layer on the substrate surfaces was removed. Next, as theIII-nitride semiconductor layer 120, the following layers were formedsuccessively onto the principal face of the III-nitride crystalsubstrates 100: a 10 nm-thick undoped Al_(0.26)Ga_(0.74)N spacer layer121; a 20 nm-thick Si-doped n-type Al_(0.26)Ga_(0.74)N carrier supplylayer 122; and a 20 nm-thick Si-doped n-type GaN contact layer 123. Theformation of the III-nitride semiconductor layer 120 was carried out byMOCVD with the substrate temperature being brought to 1,100° C.,utilizing: a trimethyl gallium and trimethylaluminum gases asIII-element source gas; an ammonia gas as nitrogen source gas; ahydrogen gas as carrier gas; and a silane gas as doping gas.

Next, as a source electrode 126 and drain electrode 127, Ti/Al layerswere evaporated respectively to thickness of 25 nm/150 nm onto theSi-doped n-type GaN contact layer 123, and were alloyed by heattreatment. After they were alloyed, the central portion of the wafer wasrecess-etched by RIE into the form of a stripe. Subsequently, a Pt layer(10 nm in thickness)/Ti layer (40 nm in thickness)/Au layer (100 nm inthickness) were formed as a gate electrode 125 onto the Si-doped n-typeAl_(0.26)Ga_(0.74)N carrier supply layer 122 exposed by the aboverecess-etching, between the source electrode 126 and the drain electrode127. In the gate electrode 125, the gate length and width were maderespectively 2 μm and 14 μm, and the interval between the sourceelectrode 126 and the drain electrode 127 was made 10 μm.

Being fabricated on the GaN crystal substrate (III-nitride crystalsubstrate 100) whose surface was planar, and was low in dislocationdensity, the HEMTs had an extremely sharp interface, and observing theinterface profile under a transmission electron microscope (TEM) provedthe interface profile to be planarized to a level of atomic layer. Theproperties of the HEMTs at a room temperature (for example, 300 K) were160 mS/mm transconductance and 1.1 A/mm drain electric current, whichmeant that the devices of highly enhanced properties could befabricated.

Embodiment 7 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 8, GaN seed crystal that was 2 inches (50.8 mm)in diameter and 350 μm in thickness, having a matrix 1 s with adislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Herein, the inversion domains 1 t were in the form ofa honeycomb whose width W was 50 μm, and whose central lines lay on thesix sides of closed-packed regular hexagons 1 h two-dimensionally alongthe (0001) plane, having sides L of 300 μm. Furthermore, the orientationof all six sides paralleled the <11-20> directions.

Additionally, the undersubstrate in Embodiment 7 was produced by, basedon the growth method set forth in Patent Document 1, growing GaN crystalby HVPE onto a GaAs substrate in which mask layers were formed on ahoneycomb whose width W was 50 μm, and whose central lines lay on thesix sides of the close-packed regular hexagons 1 h two-dimensionallyalong the (111) A-face, having sides L of 300 μm. Herein, theorientation of any one pair of facing sides of the above regularhexagons paralleled the <−110> directions in the GaAs substrate. Thatis, the inversion domains 1 t of the GaN crystal were formed onto themask layers in the GaAs substrate, and the matrix 1 s was formed ontothe GaAs substrate except where the mask layers were formed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal (III-nitride crystal 10) was grown onto the aboveundersubstrate 1 by solution deposition for 200 hours under the sameconditions as in Embodiment 1, and the GaN crystal (III-nitride crystal10) united with the undersubstrate 1 in the same manner as in Embodiment1 was picked out. The GaN crystal thickness was 6 μm.

(3) III-Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a low 1×10⁴ to 1×10⁵ cm⁻² uniformly across the crystal surfaceglobally.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 7 was crystal, in GaNcrystal of 6 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

Embodiment 8 (1) Undersubstrate Preparation

Referring to FIGS. 1 and 9, GaN seed crystal that was 2 inches (50.8 mm)in diameter and 350 μm in thickness, having a matrix 1 s with adislocation density of 1×10⁴ to 1×10⁵ cm⁻² and inversion domains 1 twith a dislocation density of 1×10⁸ to 1×10⁹ cm⁻² was prepared as anundersubstrate 1. The surface of the matrix 1 s of the undersubstrate 1was the (0001) plane, and the surface of the inversion domains 1 t wasthe (000-1) plane. Herein, the inversion domains 1 t were in the form ofa honeycomb whose width W was 50 μm, and whose central lines lay on thesix sides of close-packed regular hexagons 1 h two dimensionally alongthe (0001) plane, having sides L of 300 μm. Furthermore, the orientationof any of the six sides all paralleled the <1-100> directions.

Additionally, the undersubstrate in Embodiment 8 was produced by, basedon the growth method set forth in Patent Document 1, growing GaN crystalby HVPE onto a GaAs substrate in which mask layers were formed on ahoneycomb whose width W was 50 μm, and whose central lines lay on thesix sides of the close-packed regular hexagons 1 h two-dimensionallyalong the (111) A-face, having sides L of 300 μm. Herein, theorientation of any one pair of facing sides of the above regularhexagons paralleled the <11-2> directions in the GaAs substrate. Thatis, the inversion domains 1 t of the GaN crystal were formed onto themask layers in the GaAs substrate, and the matrix 1 s was formed ontothe GaAs substrate except where the mask layers were formed.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal (III-nitride crystal 10) was grown onto the aboveundersubstrate 1 by solution deposition for 200 hours under the sameconditions as in Embodiment 1, and the GaN crystal (III-nitride crystal10) united with the undersubstrate 1 in the same manner as in Embodiment1 was picked out. The GaN crystal thickness was 6 μm.

(3) III-Nitride Crystal Evaluation

The GaN crystal (III-nitride crystal 10) uniting with undersubstrate wasobserved under an optical and fluorescence microscopes after the sameetching as in Embodiment 1, which proved that the entirety of thecrystal surface was the Ga face ((0001) plane), and had a singlepolarity. Moreover, observation by CL revealed that dislocation densityin the GaN crystal (III-nitride crystal 10) uniting with undersubstratewas a low 1×10⁴ to 1×10⁵ cm⁻² uniformly across the crystal surfaceglobally.

From the above-noted results it could be confirmed that GaN crystalunited with an undersubstrate of Embodiment 8 was crystal, in GaNcrystal of 6 μm thickness, in which at least the surface had a singlepolarity, and the dislocation density was low globally.

Embodiment 9 (1) Undersubstrate Preparation

The same undersubstrate as in Embodiment 1 was prepared.

(2) III-Nitride Crystal Growth by Liquid-Phase Technique

GaN crystal (III-nitride crystal 10) was grown onto the aboveundersubstrate by solution deposition for 200 hours under the sameconditions as in Embodiment 1, and the GaN crystal (III-nitride crystal10) united with the undersubstrate 1 in the same manner as in Embodiment1 was picked out. The GaN crystal thickness was 6 μm.

(3) III-Nitride Crystal Growth by Vapor-Phase Technique

Referring to FIGS. 11A and 11B, the GaN crystal (III-nitride crystal 10)grown onto the undersubstrate 1 by solution deposition to unite with theundersubstrate 1 was further grown for 35 hours by HVPE to furtherthicken the GaN crystal by 3,500 μm. Herein, as dopant gas, a SiCl₂H₂gas was utilized.

(4) III-Nitride Crystal Substrate Fabrication

Referring to FIGS. 11B and 11C, the GaN crystal (III-nitride crystal 10)grown onto the undersubstrate 1 by solution deposition, and subsequentlyby HVPE to unite with the undersubstrate 1 was sliced into eight with aslicer. The eight slices of the GaN crystal were each polished from theboth faces grown by solution deposition or by HVPE,*¹⁰ and furthermore adamaged layer on the Ga face side was removed by RIE. In this manner,eight 355 μm-thick GaN crystal substrates (III-nitride crystalsubstrates L1, V1, V2, V3, V4, V5, V6, and V7) were produced. Herein,the substrate L1 was produced from the undersubstrate 1 and from theregion grown by a liquid-phase technique (that is, the first region 1 sand second regions 1 t), and the substrates V1, V2, V3, V4, V5, V6, andV7 were produced from the region grown by a vapor-phase technique (thatis, from the vapor-deposited region 10 v). Herein, the region grown by aliquid-phase technique in the substrate L1 has thickness of 5 μm.

As a result of evaporating an electrode onto the substrates' Ga faces tocarry out Hall measurement, all of the substrates L1, V1, V2, V3, V4,V5, V6, and V7 were proven to have resistivity of 0.01 Ω·cm, and wereconfirmed to be conductive substrates suitable for blue-violet laser andother optical devices.

(5) LD Fabrication

The substrates L1, V1, V2, V3, V4, V5, V6, and V7 were utilized asIII-nitride crystal substrates 100 to fabricate 1,000 LDs from each ofthe substrates in the same manner as in Embodiment 1. Carrying out thesame life test as in Embodiment 1 on the LDs proved that as to all thesubstrates, qualified-LD yield was a high 80%.

The embodiments and implementations that have been disclosed here areillustrative by nature are should not be regarded as limiting. The scopeof the invention is defined by its claims rather than the foregoingdescription, and should be understood to include the features of theclaims of the invention and equivalents thereof, in addition to allchanges falling within the scope of the claims.

1: A III-nitride crystal manufacturing method, including: a step ofpreparing an undersubstrate containing a III-nitride seed crystal, theIII-nitride seed crystal having a matrix, and inversion domains in whichthe polarity in the <0001> directions is inverted with respect to thematrix; and a step of growing a III-nitride crystal onto the matrix andinversion domains of the undersubstrate by a liquid-phase technique;characterized in that a first region, being where the growth rate ofIII-nitride crystal growing onto the matrix is greater, covers secondregions, being where the growth rate of III-nitride crystal growing ontothe inversion domains is lesser. 2: The III-nitride crystalmanufacturing method set forth in claim 1, characterized in that in theundersubstrate, the surface of the inversion domains is recessedrelative to the matrix surface. 3: The III-nitride crystal manufacturingmethod set forth in claim 1 or claim 2, characterized in that along a{0001} plane on the undersubstrate, the inversion domains are in theform of a plurality of stripe regions, with the striped regions beingarranged parallel to each other at regular intervals. 4: The III-nitridecrystal manufacturing method set forth in claim 1 or claim 2,characterized in that along a {0001} plane on the undersubstrate, theinversion domains are in the form of a plurality of dot regions, withthe dotted regions being arranged two-dimensionally at regularintervals. 5: The III-nitride crystal manufacturing method set forth inclaim 1 or claim 2, characterized in that along a {0001} plane on theundersubstrate, the inversion domains are in a honeycomb form, arrangedtwo-dimensionally in closed-packed regular hexagons. 6: The III-nitridecrystal manufacturing method set forth in claim 1 or claim 2,characterized in that the III-nitride crystal is grown to a thickness of1 μm or more. 7: The III-nitride crystal manufacturing method set forthin claim 1 or claim 2, characterized in that the resistivity of theIII-nitride crystal is in its surface is 1×10⁵ Ω·cm or more. 8: TheIII-nitride crystal manufacturing method set forth in claim 1 or claim2, characterized in that the III-nitride crystal is grown in a nitridereaction vessel. 9: The III-nitride crystal manufacturing method setforth in claim 1 or claim 2, including a step of causing III-nitridecrystal that has been grown onto the undersubstrate by a liquid-phasetechnique to grow further by a vapor-phase technique. 10: A III-nitridecrystal substrate obtained by processing III-nitride crystal produced bya manufacturing method set forth in claim 1 or claim
 2. 11: TheIII-nitride crystal substrate set forth in claim 10, wherein theprocessing of the III-nitride crystal includes a step of slicing orcleaving the III-nitride crystal. 12: A III-nitride semiconductor devicein which an at least single-lamina III nitride semiconductor layer isformed onto the III-nitride crystal substrate set forth in claim 10.